An interlaced video signal is a common format for video information. In an interlaced video signal, each image or frame is defined by a set of lines of pixels. These lines are separated into two fields such that adjacent lines in the image or frame are in separate fields. Odd lines define an odd field; even lines define an even field. In an interlaced video display, only one field of each image is displayed each time the display is refreshed, alternating between odd and even fields within each image. Because the two fields usually closely resemble each other, and because the fields are displayed at twice the frame rate, a human eye generally does not perceive that each field actually is refreshed at the frame rate and thus does not perceive flickering.
Although an interlaced video signal generally provides good quality images and sufficient bandwidth, various artifacts may occur when a time varying effect is added to the interlaced video, particularly if the effect involves positioning an image, such as a progressive scan image, into the interlaced video. An input image may be positioned into the interlaced video signal by selecting, for each line in the output image, a corresponding line in the input image and a fraction indicating a subpixel position (or offset) into that line and the next (or previous) line in the input image. The intensity, color and brightness of each pixel in the output line is determined as a function of the subpixel position and the corresponding pixels in the selected lines in the input image. Thus, two lines in two different fields in the input image typically contribute to each line in an output field. This technique may be called "unfiltered subpixel positioning." With unfiltered subpixel positioning, various artifacts may occur. Such artifacts include detail areas that appear to flicker (called twitter), such as with credits or other small text, or detail areas that appear and disappear over time.
The artifacts caused by unfiltered subpixel positioning may be reduced by positioning the input image into the interlaced video by determining the intensity, color and brightness of each pixel in the output line as a function of the subpixel position and the corresponding pixels in the corresponding line in one field of the input image and both of the adjacent lines in the other field of the input image. Thus, three lines in two different fields in the input image typically contribute to each line in an output field. This technique may be called "filtered subpixel positioning," and has at least two disadvantages. First, the input image becomes slightly blurred. Second, each output line requires additional processing and storage, in comparison to unfiltered subpixel positioning, due to the use of three input lines.